TLDR : Is there really any differences in the physiology of training muscles at 3,5,8 and 10 reps? What are the theories/evidence for this?
Over the past half year or so I have really been pushing myself in the gym, going 6 times a week, working mainly using a simple bodybuilding split. Focusing mainly on compound movements and using rep ranges of 8-10. The reason I chose this range was because in one of lectures about strength and conditioning 8-10 reps was described as the range for optimal hypertrophy.
I have been experiencing good gains in my upper body, mainly traps and lats but have been lacking growth in my legs. This lead me to ask for help from a trainer at my gym who has given me a workout plan with three full body workouts per week. He has also chosen 10 reps.
However, before launching into my new program I just wanted to ask if anyone knew the reasoning behind using 10 reps for hypertrophy. From what I have learnt muscles are composed of different types of fibers. The ones responsible for hypertrophy are the fast twitch fibers which grow in size over time when lead to fatigue and allowed to rest with adequate nutrients.
I realize this is very simplified, and there is the activity of different hormones that also has an effect. However, what I don't understand is why there would be a difference in muscle growth or strength gains when using rep ranges of 3 or 8 if the fast twitch fibers are fatigued in both cases (using different %1repmax for each rep range). In both cases the weights are lifted explosively with the concentric emphasized.
I know a lot of what is done in the gyms is more tried and tested rather then scientifically proven but I was just wondering if there was any evidence for the perceived differences.